Observations that indicate a chemical change

 Observations that indicate a chemical change

• Heat or light is absorbed or given out during a chemical reaction.
• Production of sound
• Production of gases or precipitates
• Production of smell
• A colour change may occur

Rusting

• When substances made of Iron are exposed to oxygen and moisture in the atmosphere, it forms a red layer, which is called rust.
• The formation of rust can be represented by the following reaction:
• 4Fe + 3O2 → 2Fe2O3. The chemical formula for rust is Fe2O3.nH2O. More the moisture in the air, quicker the formation of rust

Galvanization

• – The process of depositing zinc on the surface of Iron to prevent rusting is called as galvanisation.
• – Example: Iron water pipes are galvanised. Ships are made out of iron which is galvanised. Due to the presence of salts in seawater, the process of rusting is hastened. Hence ships need to replace their iron body every year.

Crystallisation
– The process of separation of salts from their solution is called as crystallisation. It is a purification technique that purifies seawater or separates crystals from impure samples. It is a physical change.